I have a layer to which I have added a layer mask. Now I select a black brush and paint over it with 90% opacity to hide that part of the layer and reveal the layer below which is grey. If I now paint again over the same area using the same brush, the effect sort of adds up and I now see that area as black instead of grey.

Is there any way at all that I can avoid this? Is there any setting with which the brush will not paint the part that I have already painted?

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There are always multiple ways to accomplish anything in Photoshop. Can you try to explain what the end effect is you are trying to accomplish? That might help us understand your question a little more, and give you an answer to get you to the end result. Also, it would be helpful if you could provide a screenshot for an example.
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Joel GlovierJul 13 '11 at 5:03

Hi Tushar. Are you editing a photograph, or is what you are doing more graphic/print design than photographic editing? If its the latter, I can move this question to our Graphic Design site here on SE, where you might get better answers.
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jristaJul 13 '11 at 22:35

3 Answers
3

Rather than using opacity of the brush, you should choose shades of gray for your brushes. That way overlapped strokes will not add up and get darker.

You can also use "Lighten" and "Darken" blend modes on those brushes to avoid messing up other parts of the mask

For example, let's say you painted part of the mask with pure black, and it's just how you want it. Now you want to paint some of the rest of the mask with an 80% gray colour. Set the blend mode of the gray brush to Darken, and it will darken anything you paint on that is still white (or ligher gray) but will not affect any parts of the mask that are black (or darker than 80% gray)

If you've already done it another way and have a few dark patches you want to fix up, select the color you want with the eye dropper and then paint that color over the darker areas you didn't want darker.

If there are harsh boundaries between light and dark, you could even try blurring the mask with gaussian blur to smooth the transition.

EDIT: what I meant is that instead of using a black brush with 90% opacity, use 90% gray brush (following your comment I understand that the opacity is applied to the brush rather than the layer, but the same advice holds here). With the black brush, multiple strokes on the same place essentially change the color (90% x 90% etc.). With the opaque gray brush, the subsequent stroke is not additive, but rather replaces the previous color, but in this case the new color is the same as the old one so you end up with the pixels unchanged.

Sorry I didn't exactly get you. It would be easier to understand if you could list the steps. The opacity I have mentioned is that of the brush and not of the layer.
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Tushar MJul 13 '11 at 4:50

ysap means this: if you paint 90% opacity black brush on white, this is sort of equivalent to 90% gray at 100% opacity. The difference is that the semi-transparent brush will not cover what is below it, where the totally opaque gray brush will.
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horatioJul 15 '11 at 21:08